bushwhacker (n.)

also bush-whacker, 1809, American English, "woodsman, one accustomed to life in the bush," literally "one who beats the bushes" (to make his way through), perhaps modeled on Dutch bosch-wachter "forest keeper;" see bush (n.) + whack (v.).

Among Northern troops in the American Civil War, "Confederate irregular who took to the woods and fought as guerrillas" (1862). Related: bushwhack (v.), 1837; bushwhacking (1826).